1. Field of Invention
This invention relates to an absolute position optical displacement-sensing device that senses absolute positions in two dimensions.
2. Description of Related Art
Absolute position encoders use a scale structure that allows the position of a readhead relative to a scale to be determined without requiring any reference to a given starting point along the scale. A variety of 1-dimensional (1D) absolute position encoders are known that use multiple information patterns extending in parallel along a 1-dimensional scale. Also, 1D and 2-dimensional (2D) codes have been used to identify positions along 1D scales. However, the structures of such absolute position encoders that are fundamentally 1-dimensional are generally not adaptable or combinable to provide a high resolution and high accuracy absolute position measurement at an arbitrary position in a two-dimensional plane.
A two-dimensional (2D) incremental (non-absolute) position encoder providing high resolution and high accuracy at an arbitrary position in a 2D plane is disclosed in U.S. Pat. No. 5,104,225 to Masreliez. However, the grating scale and readhead disclosed in the 225 patent are not adaptable to provide absolute position measurement.
Numerous 2D bar code systems are known. However, the “information storage” structures of such 2D bar code systems are generally not well suited to act as a 2D scale for high-resolution absolute position determinations. Furthermore, it is not clear how to array such codes into a continuous 2D scale and reliably discriminate between adjacent codes.